When a hotel gives a description of the rooms in the facility, which sentence is correct and why?
All the rooms are equipped with a free internet connection.
All rooms are equipped with a free internet connection.
In ordinary speech and writing, I would say "all the rooms".
In a promotional context "all rooms" is common.
Both are correct, because both are idiomatic shortenings of grammatically correct sentences. The elided words are clear from the context. The sentence
is a concise expression of the more verbose sentence
Likewise, the sentence
is a shortened version of the sentence
There are other possible "source" sentences, but to me, these seem to be the most likely candidates.